The present invention relates to a side-reinforcing rubber composition, which has excellent run flat performance, and a run flat tire using the same.
At present, a run flat tire having a side-reinforcing rubber of high hardness positioned on the inside of the sidewall has been realized and such a tire can run for a certain distance even when air pressure is lost due to puncture of the tire. As a result, a spare tire does not always need to be carried and the weight of the vehicle as a whole can be reduced. However, the speed and distance that can be run by a run flat tire when punctured is limited and further improvement in durability of a run flat tire is desired.
An example of an effective means to improve durability of a run flat tire is the method of suppressing deformation by thickening the reinforcing rubber, thereby preventing damage due to deformation. However, because the weight of the tire increases, this counters the initial purpose of a run flat tire, which is to become lightweight.
Another example of an effective means to improve durability of a run flat tire is the method of increasing the hardness of the reinforcing rubber by compounding an increased amount of a reinforcing filler such as carbon black, thereby suppressing deformation. However, improvement of run flat durability is not satisfactory, as the load to steps such as kneading and extruding is large and also, heat generating properties become high after vulcanization.
A further example of an effective means to improve durability of a run flat tire is the method of increasing vulcanization density, without increasing the amount of carbon black, by using a large amount of a vulcanizing agent and a vulcanization accelerator, thereby suppressing deformation and heat generation (for example, see JP-A-2002-155169). However, elongation of the rubber is small and strength at break decreases.
Also, improving air permeability resistance and appearance by compounding laminated natural minerals such as micas in rubber for the sidewall of a pneumatic tire has been suggested (for example, see JP-A-2003-292685 and JP-A-11-348513). However, because these rubbers require bending resistance, there is the problem that these rubbers have low hardness, which is insufficient for using as side-reinforcing rubber for a run flat tire and supporting load under low inner pressure conditions.